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Protective effect of endolithic fungal hyphae on oolitic

limestone buildings

Nicolas Concha Lozano, Pierre Gaudon, Jacques Pagès, Gisel de Billerbeck,

Dominique Lafon, Olivier Eterradossi

To cite this version:

Nicolas Concha Lozano, Pierre Gaudon, Jacques Pagès, Gisel de Billerbeck, Dominique Lafon, et al.. Protective effect of endolithic fungal hyphae on oolitic limestone buildings. Journal of Cultural Heritage, Elsevier, 2012, 13 (2), pp.120-127. �10.1016/j.culher.2011.07.006�. �hal-00782521�

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Protective effect of endolithic fungal hyphae on oolitic limestone buildings

Nicolas Concha-Lozano a, Pierre Gaudon a, Jacques Pages b, Gisel de Billerbeck c, Dominique Lafon a, Olivier Eterradossi a

a CMGD, école des Mines-d’Alès, 6, avenue de Clavières, 30319 Alès cedex, France

b Association mousses et lichens du Haut-Languedoc, Hameau-La-Gineste, 34610 Rosis, France c Laboratoire Elios, 2, rue Crébillon, 30900 Nîmes, France

Original article :

N. Concha-Lozano, P. Gaudon, J. Pages, G. de Billerbeck, D. Lafon, O. Eterradossi. Protective effect of endolithic fungal hyphae on oolitic limestone buildings. Journal of Cultural Heritage. (2012) 13(2):120–7

Keywords: Stone / Monument / Durability / Lichens Patina / Capillary / Gypsum

Abstract

This study presents characterizations of weathering forms of the same oolitic limestone from four quarries and eight monuments exposed on various environmental conditions focusing on the water-proofing effect of endolithic organic matter. Patinas were analyzed by X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX), capillarity coefficient through weathered and unweathered sides, gypsum content and porous network morphology by epoxy resin molding. Study of weathering forms on old quarries indicates that lichens colonization (Verrucaria nigrescens and Caloplaca aurantia) can fill the superficial porous network with a dense network of lichenised fungal hyphae. Capillary coefficient measurement on natural and calcinated samples showed that endolithic organic matter can waterproof the stone and could act as a sulfate contamination barrier. Similar endolithic organic layer due to ancient lichens growth are found on some antique monuments of the Nîmes downtown and could explain their well-preserved state, unlike decayed 19th century churches that were never colonized by lichens.

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1. Research aims

This study focuses on a recurring debate about the development of patinas and their role in the protection or acceleration of buildings deterioration [1–3]. It is commonly accepted that weathered forms–or patinas–depend on environmental factors around the exposed surfaces, such as water and aerosols exposure [4,5]. Paradoxically, in the central city of Nîmes some monuments built with the same oolitic limestone are well preserved while others are severely degraded despite of their similar environmental conditions. Two types of weathering are observed. On one hand, several monuments (e.g. 19th century churches) are remarkably damaged, suffering blistering, yellowing and granular disintegration according to ICOMOS glossary [6]. On the other hand, monuments even much older (e.g. the roman temple Maison Carrée) and located in the same neighborhood do not exhibit such severe degradations. The main objective of this study is to further understand these observed weathering paths divergences, focusing particularly on the bioprotective role of lichens. For this, microstructure, composition and water transfer properties of patinas were studied on Bois des Lens oolitic limestone samples collected on 12 different sites.

2. Introduction

Bois des Lens stone is a white and fine-grained oolitic limestone commonly used for building and sculpture over 20 centuries in southern France. Its exploitation and use as a building stone started in about the 4th BC century and its dissemination in the antic architecture extends over a large part of French Mediterranean coast (Narbonne, Beziers, Arles, Frejus, Nice) [7,8]. This early cretaceous sedimentary rock is extracted from massive outcrops that allow blocs size up to several meters. At the macroscopic scale, the stone has a smooth feel, bright uniform color and invisible bedding. Due to its isotropic mechanical properties the stone is appreciated for ornamental architecture and sculpture. Its relatively high porosity (inter-and intra oolitic voids) does not allow a glossy polish but a fine softened surface. The peculiarity of this stone is that its patina extends up to several millimeters in depth, which is the cause of a great diversity of appearance in terms of color and texture after aging [9]. Patinas can be composed by several layers like deposits and sub-surfaces modifications depending on reversible or irreversible weathering mechanisms [10]. Surface deposits layers are composed by air born particles, aerosols, salts precipitation, or epilithic bio-logical colonization weakly adherent to the mineral substrate and easily removable. Sub-surface modifications are

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described as the location of irreversible aging mechanisms like structural and com-position change. In this study, the patina is defined as the layer affected by irreversible aging that cannot be removed by conventional cleaning processes such as laser or sandblasting. Decrease of erosion rate due to protective microorganism inter-face between the stone and its environment is an example of the bioprotection concept [11]. According to Carter and al. [12], bioprotection is a little know-earth surface processes in comparison with the vast literature on lichen biodeterioration. Although deterioration mechanisms by lichens are well known [13,14], it is unclear whether the weathering rate would be lower without them, especially in a polluted urban environment. Recent field studies gave evidence of the protective effect of lichens [15–17] highlighting that biodeterioration is slower than physicochemical process. The main bioprotection mechanism is often called “umbrella effect” illustrating that lichens thallus forms a barrier layer that reduces the amount of runoff water in contact with stones [12,17], but also protects the surface from wind erosion and reduces thermoclastic damaging due to intermittent solar radiation [2,18]. This study focuses on a second type of bioprotection mechanism, due to the presence of organic matter entrapped beneath the surface of the stone, even after complete removal of epilithic biological colonization by cleaning.

3. Materials and methods 3.1. Sampling method

All samples were collected from monuments and quarries of Languedoc-Roussillon region, located in the South of France. To ensure that samples have a similar lithology, a previous literature review was conducted to identify monuments and quarries of Bois des Lens stone. An initial list of 13 monuments and one quarry was extracted from the MONUMAT database [19]. MONUMAT database is a tool developed by the French geological survey (BRGM), which contains an inventory of the main historic monuments, the stones used for their construction and location of the quarries. This information is accessible for all users involved in historical heritage conservation (architect, local authorities, companies specializing in conservation, etc.) through a web interface [19]. Among the initial list of buildings identified, only eight were selected after petro-graphic verification and their potential for sampling (Table 1). The localization of the three antique quarries (Rocamat antique, Pielles quarry and Roquet quarry) was suggested by the work of archaeologist Bessac who studied the history of extraction techniques of Bois des Lens stone [7,8]. The quarries are located on the oolitic

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facies identified under n4bU (Barremian age) according to the BRGM geo-logical map of Sommière [20]. The sampling of the 19th century churches was done with a scalpel on representative areas according to procedures used in previous studies [21,22]. Samples of the Maison Carrée (MC1), come from the western facade and were collected as the opportunity of a restoration project. Sample MC1-3 was collected on the internal side of removed rubble and considered as an unweathered reference state (Table 1).

Tab. 1, Sampled sites of Bois des Lens oolitic limestone with their location, age, and car traffic exposure indication.

Samples Sites Locality Years of exposition Traffic exposition

MC1 Maison Carrée Nîmes Ist Century High

MC1-2 Maison Carrée Nîmes Ist Century High (sanded) MC1-3 Maison Carrée Nîmes Ist Century Not exposed TD3 Temple of Diana Nîmes Ist Century High RQ2 Roquet quarry Moulezan Roman Low PI3 Pielles quarry Combas Roman - XVth Low ROA Rocamat antique Moulezan Roman Low SG2 Abbey of St Gilles St Gilles 12th Century Medium EB3 ST Baudile church Nîmes 1867 - 1877 High EPO Pompignan church Pompignan 1850 Medium MMF War memorial Fons 1920 Low EP2 St Perpetue church Nîmes 1852 - 1862 High SP2 St Paul church Nîmes 1835 - 1849 High RA Rocamat quarry A Moulezan 2009 low RB Rocamat quarry B Moulezan 1996 Low RC Rocamat quarry C Moulezan 1990 ± 2 Low RD Rocamat quarry D Moulezan 1960 ± 10 Low

RD-2 Rocamat quarry D Moulezan 1960 ± 10 Low (sanded)

3.2. Physicochemical characterizations

Each sample was sawn with a diamond disc perpendicular to the exposed surface. Samples were immersed in epoxy resin under vacuum and then finely polished for examination with an environmental scanning electron microscope (QUANTA 200 FEG, FEI Company). Petrographic analysis of minerals, grain joints and size was conducted by crossed nicols optical microscopy (LEITZ Labor-lux 11 POL S) on 30 µm thin section. For mineralogical and chemical analysis, we used blades of rock obtained by microtome or powder obtained by rasp. Mineralogical composition was analyzed by X-rays diffraction (BRUCKER AXS D8 ADVANCE) on first 4 mm powdered patinas. The distribution of sulfur and calcium has been mapped on the first 2 mm with EDX (INCA X-ray microanalysis). Distribution of sulfur was

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mapped in order to compare the gypsum contamination depth through the patina with a similar procedure adopted in previous studies [1]. Gypsum content (wt. %) was estimated from electric conductivity (JETWAY 4510) of unsaturated solution of powdered first 4 mm patinas in deionized water using a calibration curve. The electric conductivity calibration curve was obtained from synthetic solutions saturated in respect to calcite with several amounts of solubilized gypsum assuming that the solubility of calcite (Ksp = 9.8 × 10−9) is negligible compared with that of gypsum (Ksp = 2.4 × 10−5) and that gypsum is the only soluble salt (supported by XRD and EDX measurement). Lichen genus was identified by binocular microscopy (LEICA WILD M10) on hymenial layer extracted from mature fungal fructification. Fruiting bodies were extracted from patinas using a pin and then cut in half by a razor blade.

Tab. 2, Uniformity of the Bois des Lens stone in spite of the different sampling location. Petrophysical properties of unweathered samples in the commercial quarry (RA), on antique quarrys (PI3 and RQ2) and on the Maison Carrée (MC1-3).

Petrophysical properties RA PI3 RQ2 MC1-3

Minerals Calcite, Quartz Calcite Calcite, Quartz Calcite, Quartz Ca (% atom.) 19,9 20,0 20,0 20,0 C (% atom.) 19,8 19,9 19,4 19,5 O (% atom.) 59,6 59,8 59,1 59,2 Si (% atom.) 0,6 0,3 1,3 0,9 Other Elements (% atom.) 0,0 0,0 0,3 0,4 Density (g.cm3) 2.2-2.3 2.23 2.26 2.21 Total porosity (%) 13-17 15-16 15-17 15-16 Water accessible porosity 11-14 13-13 12-14 13-13 Capillary coef. (g.cm-2.s-0.5) 60-90 82-79 68-75 70-72

3.3. Pore network molding

As the stone is composed of almost pure calcium carbonate, a molding of porosity was performed by resin impregnation and calcite crystals dissolution. Dried samples were impregnated with epoxy resin under vacuum (around 10−2 Pa). After resin polymerization, samples were finely polished and then immersed in a 30% HCl solution. Once the dissolution reaction was completed, the samples were extensively washed in pure water and then dried in open air. Total dissolution of calcite was confirmed by EDX calcium quantification performed on molded samples.

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3.4. Water transfer properties

Test samples were cut into cuboids of 1.5 × 1.5 × 4 cm with one weathered side. Two samples were sandblasted (quartz sand under 4 bar pressure) in order to remove the superficial biological colonization (on sample RD-2) or any traces of lime whitewash that could be applied during previous restoration work (on sample MC1- 2). Capillary coefficients were measured four times for each sample with deionized water: first on weathered and unweathered faces and then after calcination on both sides. Calcination was performed in an oven at 500 ◦C for 20 min in order to remove the intraporous organic matter. Standard capillary measurement protocols (e.g. EN 1925) were not suitable for measurement through thin stratified materials like patinas (< 1 mm) due to the fact that contact between the sample and the free water must be precisely controlled to pre-vent absorption of water through the unweathered sample sides. So, an automatic monitoring soaked volume apparatus was used (KSV INSTRUMENT LPR 902) whose scheme is shown in Fig. 1. Capillarity coefficient C [g m−2 s−0.5] was calculated using:

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with Vt the volume soaked at the time t through the surface S. Imbibitions duration was 30 min with an acquisition frequency of 1 Hz. Sample were dried in an oven at 60 ◦C for 12 h before each capillarity measurement. To reach a comparable initial saturation index, calcinated samples were previously immersed in water and dried under the same conditions than non-calcinated samples.

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Fig. 1. Scheme of the used automatic monitoring soaked volume for capillarity measurement (KSV INSTRUMENT LPR 902). Here the sample is positioned for the weathered side capillary measurement.

4. Results and discussions

4.1. Petrophysical properties of unweathered Bois des Lens oolitic limestone

Table 2 summarizes some petrophysical characteristics of four samples of different provenances (three quarries and one monument) in order to estimate the natural variability of the Bois des Lens stone facies. The petrophysical properties uniformity at the initial state (unweathered) is a necessary condition for a comparative aging study. Unweathered Bois des Lens stone is composed of quasi-pure calcium carbonate. Only calcite and traces of quartz are detected by DRX. However, quartz content is low with respect to calcite since less than 1.3% atom are silicon (Table 2). Thin sections show well-sorted rounded oolites whose diameter ranges from 0.2 to 0.6 mm. Oolites nucleus are composed by foraminifer-ous fragments and surrounded by a layered micritic cortex (Fig. 2). Oolites are weakly compacted and cemented by a micritic matrix with some sporadic sparite crystals. Imbibition test give an open porosity of 11–14% and a total porosity of 13–17% (Table 2). Although there are slight variations in petrographic properties, studied samples can be considered homogenous despite their different sampling location.

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Fig. 2. Ooide grainstone texture of Bois des Lens cretaceous limestone viewed by transmission microscope on a thin section, sample (MC1). S: exposed surface. Note that evidence of endolithic biological colonization is not visible.

4.2. Weathering forms of well-preserved building

The patina of the Maison Carrée is a characteristic example of low erosion and good mechanical cohesion weathered form. The patina is composed of several layers beneath the stone sur-face (Fig. 3a). The deepest layer located from 3 to 0.5 mm deep is a brownish or sometimes greenish layer. It’s boundary with unaltered rock is diffuse and is always associated with a dense hyphae network (Table 3). Analysis of thin sections shows that the weathered zone is not a deposit or an encrustation but a superficial transformation since oolitic structure is visible from the unweathered area until the exposed surface (Fig. 2). Nevertheless, the oolitic grain density seems to be lower at 1 mm deep, which implies a change in the petrographic texture and supports the existence of endolithic dissolution of calcite. As shown in Fig. 4c, the molded porosity shows a high density of micrometric tubes. This structural transformation of the rock could be the result of a dissolution/ precipitation activity of endolithic lichen hyphae. A calcite structure is still visible even in the most invaded area (voids in Fig. 4c) that contributes to the mechanical cohesion of the patina [27]. Nevertheless, this layer has lost its microporosity visible on the initial oolitic cortex. This implies that all the pores are filled with hyphae. Above the brownish layer, a thinner and lighter layer was observed which structure is petrographically nearly identical to that of the unweathered side. Although this superficial layer is thin (about 500 microns) it gives an unweathered appearance whereas the patina is deeply invaded by biological colonization. A deposit rich in sulfur from about 100 microns thick covers the patina. This deposit is the cause of the Maison Carrée blackening. The origin of sulfur is attributed to an urban source of

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contamination since monuments located in rural area (defined as an area of low car traffic) do not contain gypsum (Table 5). Table 3 shows that patinas sampled in quarries or on well-preserved buildings share some common points. For example, macroscopic observations on sample MC1 and RD show that both have the same indurated brownish layer (Fig. 3a and b). The MEB images of pore resin molding (Fig. 4c) point out that brownish layer is associated with the presence of lichen hyphae. Likewise, for all other samples, the lichen hyphae layer is the discriminating attribute between the protective and deteriorated patinas (Table 5). In quarries, stones of more than 20 years of exposure are firstly covered with black lichen Verrucaria nigrescens (Fig. 5) and then a second type of lichen Caloplaca aurantia appears on samples exposed for over 50 years (Table 3).

Fig. 3. Microphotograph of a weathered side profile (antique quarry RQ2) showing the oolitic limestone interface with Verrucaria nigrescens lichen fructification.

4.3. Weathering forms of deteriorated buildings

A link can be established between the gypsum content and the appearance of blisters and crumbling. All deteriorated buildings contains high load of gypsum upper than 15% wt. in their first 4 mm (sample EB3, EPO, and SP2, in Table 5). Below this gypsum content, the stones do not desquamate, and no erosion of their surface is visible (e.g. SG2, EP0, TD3 and MC1, in Table 5). However it is necessary to distinguish between the superficial deposits of

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gypsum (as a black crust of Maison Carrée, MC1) and the deep contamination as the churches of Nîmes (EB3, EP2, SP2). In the first case (Fig. 4a and b), the gypsum is mixed with the carbonaceous material deposited in thin layers of 0,5 to 1 mm weakly adherent to the stone surface with an entry of sulfur that does not exceed 500 µm. This superficial layer is only responsible of the monument darkening but not of the mechanical decohesion. In the second case (Fig. 4d and e), microcrystals of gypsum are detected up to several centimeters deep, associated with blistering, disintegration, and yellowing. Fig. 4 shows a clear grain shape modification of the sulfur-contaminated stone (SP2, St Paul church) whose grains appear smaller than in the Maison Carrée. In addition, oolites and biodetritic elements (Fig. 4d) are no longer visible. This change may be due to carbonate replacement by gypsum crystals since limestone is the only source of calcium [23–25]. The decrease of calcite ratio also explains weakness, disintegration and swelling of the monuments, which contain gypsum ratio upper to 15% wt [26].

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Tab. 3, Description by layers of weathered forms (patinas) classified into three categories: well-preserved, damaged and aged in a quarry.

Sample Site

Description of patinas layers: fist layer

second layer third layer

Deteriorated samples

EB3 ST Baudile church Granular disintegration, yellowing Cracking, dechoesion and blistering EP2 St Perpetue church Granular disintegration, yellowing

Cracking, loss of grains cohesion and desquamation of 5mm thick plates SP2 St Paul church Granular disintegration, yellowing

Fissuration, loss of grains cohesion and blistering EPO Pompignan church Coloration (yellowing) and slight loss of cohesion

Well preserved samples

MC1 Maison Carrée

0-100 µm: superficial deposit of black gypsum crust 100 –500 µm: well preserved layer

Endolitic hyphae network (Brownish layer) MC1-2 Maison Carrée 100 –500 µm: well preserved layer

Endolitic hyphae network (Brownish layer) MC1-3 Maison Carrée No patina

TD3 Temple of Diana

0-100 µm: superficial deposit of black gypsum crust 100 –500 µm: well preserved layer

Endolitic hyphae network (Brownish layer) SG2 Abbey of St Gilles 0-300µm black/grey superficial deposit of gypsum

Endolitic hyphae network (Brownish) MMF War memorial

Indurated layer with biopitting Endolitic hyphae network (green)

Quarry samples

RA Rocamat quarry A No patina RB Rocamat quarry B Removable dust

RC Rocamat quarry C 0-100 µm Epilithic colonization : Verrucaria lichens (Black)

RD Rocamat quarry D

Epilithic colonization: Thales of Caloplaca Aurantia (orange) and Verrucaria lichens (Black)

Weddellite spots

Endolitique hyphae network (Brownish or green) zone RD-2 Rocamat quarry D Endolitique hyphae network (Brownish or green) zone

RQ2 Roquet quarry

Epilithic colonization: Thales of Caloplaca Aurantia (orange) or Verrucaria lichens (Black)

spots of 100 μm Oxalate crust (weddellite) Green algae and lichens fructification (perithès) Endolitique hyphae network (brownish or green) PI3 Pielles quarry

Epilithic colonization : Caloplaca Aurantia Weddellite spots

Dense hyphae network

ROA Rocamat antique Epilithic colonization: Verrucaria lichens (Black) 2 mm of dense hyphae network

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4.4. Waterproofing effect of intraporous organic matter

As shown in Table 4, capillary tests through unweathered sides show similar capillary coefficient with regards to the small standard deviation (4,1) over average value (75,0) ratio. This uniformity of water transfer behavior at the initial states that is a crucial pre-requisite for any comparative aging study [27]. Regarding capillary coefficient through weathered sides, a wide variation is observed in accordance with the wide diversity of patinas described in Table 3. A waterproofing index WINDEX (%) can be calculated in function of the capillary coefficients of the unweathered CU and weathered CW sides:

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An index close to 100, characterizes a highly impermeable patina, while an index close to 0 means no waterproofing effect. A waterproofing index of 0 was assigned to deteriorated patinas since their low cohesion cannot provide any protective effect. Table 5 shows that there are two types of patina according to the waterproofing index. Patinas with a low waterproofing index (< 14.6) are samples recently exposed and not covered with lichens. Patinas of the second category have a high waterproofing index (> 87) and contain organic matter although some are not covered by a biological colonization (MC1, TD3, SG2 and MMF, in Table 3). This waterproofing effect is mainly attributed to the entrapped organic matter that fills pores and leads to a hydraulic conductivity drop. This assumption is supported by the close capillarity coefficient value of weathered and weathered-calcinated sides (Table 4) since organic matter was removed by the calcination treatment. Note that the capillarity coefficient of unweathered and unweathered-calcinated sides remains unchanged, so the calcination treatment does not modify the initial capillary coefficient of the stone. The effect of dirt, dust or traces of ancient lime whitewash on waterproofing is negligible, considering that they are removed by sandblasting. Indeed, waterproofing index differences between non-sanded and non-sanded patina on samples MC1 and RD are not representative since they are 4% and 3% respectively (Table 5), which is below the accuracy of capillary measurements estimated around 5% (Table 4).

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Tab. 4, Capillary coefficient through both weathered and unweathered sides before and after organic matter removal by calcination

Capillary coefficient (g.m-2.s-1/2)

Samples Sites Unweathered

side Weathered side Weathered side after calcination Unweathered side after calcination MC1 Maison Carrée 70,50 5,3 68,4 71,2 MC1-2 Maison Carrée 71.3 8.3 (sanded) 72.6 68.4 TD3 Temple of Diana * * * * RQ2 Roquet quarry 73,4 8,2 79,8 78,6 PI3 Pielles quarry 82,3 2,6 76,2 79,9 ROA Rocamat antique 76,3 2,9 72,3 70,9 SG2 Abbey of St Gilles * * * * EB3 ST Baudile church * * * * EPO Pompignan church * * * *

MMF War memorial * * * *

EP2 St Perpetue church * * * * SP2 St Paul church * * * * RA Rocamat quarry A 72,9 72,3 73,5 77,5 RB Rocamat quarry B 70,6 60,3 67,2 68,6 RC Rocamat quarry C 78,9 9,7 72,1 79,1 RD Rocamat quarry D 75,4 5,5 76,7 72,6 RD-2 Rocamat quarry D 72,9 7,5 (sanded) 70,7 74,6

Mean 75,0 73,3 74,8

Standard deviation 4,1 4,2 4,4

* Too small sample size for capillarity measurement

4.5. Sulfur diffusion barrier Sulfur EDX mapping of Maison Carrée

Patinas (Fig. 4b) show a superficial gypsum contamination. Sulfur content decreases suddenly around 600 µm, which corresponds to the hyphae network layer. Conversely, Patinas of the St Paul’s Church is deeply contaminated and has not been colonized by lichens (Fig. 4e). The water flow across stone surface during wetting and drying cycles is the main conveyor of soluble species such as sulfate or calcium ions. Waterproofing due to the growth of lichens can reduce the mass of water flowing through the patina of the stone, and therefore, reduces soluble salts diffusion.

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Fig. 4. Comparison between a patina of well-preserved monument (left column) and a patina of deteriorated monument (right column). Sulfur distribution along the profile is modified by the hyphae network layer. (a) and (d): SEM. (b) and (e) EDX sulfur map. (c) SEM of pore molding showing a secondary porosity due to growth of endolithic lichen hyphae.

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Fig. 5. Photographs showing three different samples of Bois des Lens stone patinas. (a) Fac¸ ade of a Roman temple dating from the 1st century after JC in Nîmes downtown (la Maison Carrée) and detail of a patinated sample (MC1). (c) Front of St Paul’s church showing blistering, yellowing and granular disintegration. (b) Stone quarry currently in operation and details of a sample taken from a 50 years working face (RD).

5. Conclusion

This study consists in providing some explanations on the weathering differences of the Nîmes downtown monuments built with the same stone focusing on the protective role of organic matter trapped beneath the stone surface. Petro physical characterization of patinas sampled on monuments located in the same neighborhood leads to a classification into two main categories: yellowish patinas that become blistered and disintegrated and on the other hand, patina of well-preserved monuments that maintain a mechanical cohesion. The main pathology of decayed monuments is a loss of mechanical cohesion due to deep gypsum content, especially on the 19th century churches. Regarding the well-preserved monuments, a layer of entrapped organic matter was detected below the surface. Morphological analysis of the porous network by resin molding showed the existence of a secondary porosity filled by

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lichen hyphae. Similar endolithic colonization was observed in stones aged in quarries due to growth of Verrucaria nigrescens and Caloplaca aurantia lichens. The protective role of entrapped organic matter is supported by capillarity measurement that showed a significant pore-sealing waterproofing. Moreover, comparative sulfur content cartography indicates that the waterproofing effect appears to slow down the sulfate diffusion through the stone surface. There is evidence that lichen growth in a previous era is enough to explain the good preservation of some antique monuments of Nîmes.

Acknowledgments

This research was funded by the civil engineering laboratory of the école des Mines d’Alès. The authors gratefully acknowledge the contribution of E. Garcia-Diaz and L. Tibiletti for their helpful comments. Thank also to J. M. Taulemesse and A. Diaz for their technical support.

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[27] K. Beck, M. Al-Mukhtar, Evaluation of the compatibility of building limestones from salt crystallization

Figure

Tab.  1,  Sampled  sites  of  Bois  des  Lens  oolitic  limestone  with  their  location,  age,  and  car  traffic exposure indication
Tab.  2,  Uniformity  of  the  Bois  des  Lens  stone  in  spite  of  the  different  sampling  location
Fig. 1. Scheme of the used automatic monitoring soaked volume for capillarity measurement  (KSV  INSTRUMENT  LPR  902)
Fig.  2.  Ooide  grainstone  texture  of  Bois  des  Lens  cretaceous  limestone  viewed  by  transmission  microscope  on  a  thin  section,  sample  (MC1)
+5

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